Process for the production of boron phosphide



March 7, 1961 F. v. WILLIAMS ETAL 2,974,064

PROCESS FOR THE PRODUCTION OF BORON PHOSPHIDE Filed March 3, 1958INVENTOR. FORREST WILLIAMS BYM ATTORNEY PROCESS FOR THE PRODUCTION OFBORON PHOSPHIDE Forrest V. Williams and Robert A. Ruehrwein, Dayton,Ohio, and Therald Moeller, Urbana, [1]., assignors to Monsanto ChemicalCompany, St. Louis, Mo., a corporation of Delaware Filed Mar. 3, 1958,Ser. No. 718,464

4 Claims. (Cl. 117-106) The present invention relates to a new methodfor the production of boron phosphide as a crystalline composition ofmatter.

It is an object of this invention to provide boron phosphide, BP, in theform of a well-crystallized, hard, thermally stable and chemically inertmaterial. It is a further object to provide a highly abrasive,chemically inert form of boron phosphide which may be prepared StatesPatent O i in the form of granular crystalline particles suitable foruse as an abrasive material. It is a still further object of thisinvention to provide a process by which shaped structures such asrefractory articles, chemical apparatus, turbine blades and combustionfittings exemplified by combustion chambers and nozzles may be preparedfrom crystalline boron phosphide.'

Further objects and advantages of the invention will be apparent fromthe following description.

The improved form of boron phosphide is prepared by a reaction in thegaseous phase at elevated temperatures between a gaseous boron compound,elemental phosphorus and hydrogen. Examples of boron compounds which aregaseous under the present reaction conditions include the boron halides,e.g., boron trichloride, boron tribromide and boron triiodide; alkylboron compounds such as trimethyl boron, triethyl boron, tripropylboron, triisopropyl boron, and tri-tert-butyl boron, ethyl alkylatedpentaborane and ethyl alkylated decaborane; and boron hydrides includingdiborane, pentaborane and decaborane.

The temperature of reaction between the above-described boron compoundswith the elemental phosphorus and hydrogen will generally be above about1,l00 F. and may be as high as 3,600 F., a preferred range being from1,600 F. to 2,700 F. The time required for the reaction is dependentupon the temperature and the degree of mixing of the reactants.

The proportions of the respective three components may be variedconsiderably in accordance with the physical nature of the product whichis desired, but in general is from 1 to 1.5 moles of elementalphosphorus (calculated as the monatomic form) and from 1.5 to 5 moles ofhydrogen, with respect to 1 mole of the boron compound. Larger excessesof any component may be employed if desired. It is obvious that theboron compound may also furnish a portion of the hydrogen required, suchas in the use of boron hydrides as the boron source. The yields obtainedin the present process are quite high, approaching the theoreticalyield.

When carrying out the present process it has been found convenient tomake use of a high temperature furnace, such as a resistance orinduction furnace in which a separate jet of the boron compound, forexample, boron trichloride, is connected with a separatelyintroduced jetof elemental phosphorus and of hydrogen. It is desirable that the gasmixture or the individual gas streams undergo turbulent mixing in orderto assure completion of the reaction with the formation of the cubiccrystalline form of boron phosphide. The phoscoated with the crystallineproduct.

phorus may be vaporized directly into the system or may be introduced inan inert gas stream.

It is also an embodiment of the invention to form shaped objects ofcrystalline boron phosphide directly from the gaseous reactantscomprising the boron compound, elemental phosphorus and hydrogen.In-this embodiment of the invention a prototype of the desired shape,for example, a needle-form or blunt type nose cone for a guided missileis first formed from a thermally stable material, such as molybdenummetal. This form is placed in a high temperature furnace which can bemaintained at the desired temperature of about 1,600 F. to 2,700 E, andwhich is provided with inlet jets of boron trichloride, elementalphosphorus and hydrogen. If it is desired to premix these gases atsomewhat lower temperatures and admit such mixture as a unitary streamto the high temperature region, this modi fication is also feasible.

The reaction mixture of boron trichloride and the phosphorus andhydrogen are then directed to converge on the shaped form inthe hightemperature region. Provision may also be made if desired to rotate thesaid piece, such as by means of an external magnetic control to assurethe uniform deposition of boron phosphide on the form. It has been foundthat an adherent coating of boron phosphide in the cubic crystallinemodification is deposited upon the said form in accordance with thechemical reaction described above. After the desired thickness ofcrystalline boron phosphide has been obtained the entire system iscooled, after which the form with its boron phosphide coating is removedfrom v the furnace.

The coating, together with the prototype may be desired as a unitarystructure, such as in the provision of a coating of crystalline boronphosphide upon molybdenum. This combination gives a nose cone havinghigh temperature stability. On the other hand, if it is desired to stripor remove the said coating from the prototype, this may also beaccomplished. The external coating of crystalline boron phosphide whichis obtained either as separable shell or as a coated base in accordancewith the above procedure is an extremely hard, curved-shaped object sothat the nose piece, for example, is ready for use.

The crystalline boron phosphide may be prepared entirely in the gasstream. However, it is usuallymore efiicient to form the cubiccrystalline boron phosphide on a surface such as a prototype form whichis to be However, it is also contemplated that the boron phosphide becollected on the walls of a collecting chamber. It is found that theproduct is readily removed from the walls of the collecting chamber as acoherent material which may be further crushed if desired. The yield ofboron phosphide obtained by the above methods is quite high andapproaches quantitative yields.

The preparation of boron phosphide in accordance with the presentinvention is illustrated by the drawing of the present application inwhich the legends explain the various parts of the reaction system.

Boron phosphide as herein prepared is a highly crystalline material witha cubic crystalline structure having a unit cell length of about 4.537Angstrom units. Its hardness lies between 8 and 9 on Mohs scale(diamond: 10). It is, however, not as hard as silicon carbide, but ithas been found that it will scratch and abrade quartz, porcelain, agate,cemented tungsten carbide and possibly sapphire. The crystallinematerial is quite light, hav-. ing a particle density by the pycnometermethod of 2.94. (theoretical, 2.97). t T

The crystalline form of boron phosphide is resistant Patented Mar. 7, 1961 to oxidation when exposed to an oxy-hydrogen flame giving atemperature of 4000 F. In addition it has been found that a sample atthis temperature can be subjected to an oxygen jet from a cutting torchwithout appreciable deterioration of the crystalline boron phosphide.

While this material is somewhat less resistant to oxidation while it isbeing heated up to such high temperatures, the provision of a neutral orreducing atmosphere overcomes any such tendency towards deterioration.When exposed to a frame at 2,100" P. in air, it will not burn. A thincoating apparently forms on the exposed surface, which coating protectsthe boron phosphide at these high temperatures. The melting point ofthis material is extremely high, but from theoretical considerations andby analogy with data on similar compounds, it should melt at atemperature greater than about 5,400 F.

Cubic crystalline boron phosphide is not attacked by any liquid reagentwhich has been tried. It is completely stable to boiling nitric acid andto boiling aqua regia.

The following examples illustrate specific embodiments of the presentinvention.

Example 1 Two resistance-type electric furnaces were located to surrounda quartz tube and provide for two separate zones of temperature. Thefirst region was charged with a graphite boat containing red phosphoruswhich was maintained at about 900 to 1,000 F. in orderto vaporizeelemental phosphorus. The quartz tube was also provided with separateinlet fittings to introduce boron trichloride and hydrogen into thetube. It was found that a chemical reaction occurred between the vaporphase constituents of boron trichloride, phosphorus and hydrogen to forma cubic crystalline type of boron phosphide in the second hightemperature zone maintained at l,900 F. A cold trap was provided at theexit end of the quartz tube, but it was found that none of the boronphosphide collected therein, since the deposition of the material wascompleted in the high temperature region.

Example 2 The formation of a curved object of crystalline boronphosphide was carried out by employing a rod of molybdenum located inthe second zone of the furnace described above with the molybdenum beingmaintained at about 1,900 F. A gas mixture of 1 mole of borontrichloride, 1 mole of elemental phosphorus (calculated to be the.monatomic form) and 1 /2 moles of hydrogen was passed through thequartz tube and directed against the molybdenum form. It was found thatthe gas phase reactants underwent a chemical reaction with the formationof a layer of crystalline boron phosphide on the molybdenum form. Aftera sufficiently thick layer of boron phosphide had thus been obtained thefurnace was cooled and the molybdenum form with its coating of cubiccrystalline boron phosphide was removed from the furnace. It was foundthat the boron phosphide was chemically inert against nitric acid andaqua regia as well as an oxy hydrogen flame. However, it was found thatsintering of the coating could be accomplished when the shaped objectwas subjected to a high temperature, i.e., above about 2,700 F.

Example 3 The formation of a venturi throat for a rocket was carried outby utilizing a graphite prototype of the desired form. This form wasplaced in a furnace provided with an electric heating element whichcould be heated to a temperature of 2,000 F. Separate streams of 1 moleof boron trichloride, 1 mole of elemental phosphorus and 1 /2 moles ofhydrogen were admitted to the reaction zone. The graphite form was foundto provide a reaction surface on which the boron trichloride reactedwith the elemental phosphorus and hydrogen with the consequenttransformation to crystalline boron phosphide as a coating on thegraphite prototype. The coating process was continued until a depositionof about inch thickness was obtained. The sample was then removed fromthe furnace and was found to have a very smooth coating of crystallineboron phosphide so that the fabricated nozzle could be employed directlywithout the necessity of mechanical finishing.

The cubic crystalline form of boron phosphide is characterized byunusually high temperature stability. It has been foundthat thismaterial may be subjected for brief periods to temperatures of about6,000 F. This material is also resistant to attack by any known liquidchemical reagents, including the mineral acids, for example, sulfuricacid, hydrochloric acid and fuming nitric acid, as well as basicmaterials such as caustic and hydrazine. Aqua regia does not attack thecrystalline form of boron phosphide and an oxy-hydrogen flame directedintermittently against the crystalline product does not cause anyappreciable oxidation.

The inert character of crystalline boron phosphide as well as its hightemperature stability, makes this a useful material in the fabricationof rocket and jet fittings and hardware. Examples of some of the partswhich can thus be fabricated from crystalline boron phosphide includecorrosion-resistant combustion chambers and liners for various vessels,including fuel tanks which are to be used to store both liquid and solidpropellant fuels and oxidants, including ammonium perchlorate, fumingnitric acid and alkyl boron compounds, such as ethyl alkylatedpentaborane and ethyl alkylated decaborane. Missile elements which mustwithstand extreme abrasion and high temperature shock may also bemanufactured from crystalline boron phosphide; examples of such fittingsinclude nose cones and rocket nozzles. Other hardware items which mustwithstand the abrasion of high temperature gas streams and are thereforepreferably made from crystalline boron phosphide include jet vanes,elevators and control surfaces. It is an advantage of the presentinvention that curved shapes may readily be manufactured in a form whichis characterized by high strength. The formation of the crystallinemodification of boron phosphide results in the production of a grossstructure of the particles to provide interlocking of the crystallites.This is particularly advantageous in the fabrication of curved shapessince the interlocking of the crystallites results in the production ofa smooth, curved surface. This effect is advantageous in the fabricationof parts which must undergo great thermal stress and shock, for example,in the nose cones of rockets and missiles. The present type crystallineboron phosphide has also been found to be stable to combustion gaseswithout appreciable attack of the boron phosphide. Therefore, the curvedshapes which are made from boron phosphide are especially advantageousas combustion chambers and throats in which a rocket fuel, for example,diboraue and an oxidizing agent such as fuming nitric acid are comingledin order to provide a controlled combustion which releases a very largeamount of energy, such as in the propulsion of a rocket.

Since crystalline boron phosphide is also a very hard material having ahardness of Mobs scale between 8 and 9 (diamond=l0), this material isparticularly suitable for the manufacture of impellers for fuel pumps inmissiles, rockets and space ships and other moving parts.

The high temperature stability of crystalline boron phosphide makes thismaterial particularly valuable in the fabrication of parts for turbines,including both combustion turbines and steam turbines. Specific partsthus contemplated include the nozzles for either a steam or combustiongas stream (the latter possibly including fiy ash and metallic particlestherein) and also the turbine blades, vanes and bearings.

The inert character of crystalline boron phosphide in corrosiveatmospheres makes this material a valuable source for the manufacture ofsteam jet 'ejectors and rupture discs which must maintain their form andstrength at a constant value despite exposure to corrosive atmospheres,such as in petroleum refining.

Because of the hardness of crystalline boron phosphide, this material isespecially adapted for use as an abrasive material or cutting tool,either in the form of a finelydivided product or in a fabricated form,for example as a cutting tool in a lathe. The particulate form ofcrystalline boron phosphide may also be used in the manufacture ofgrinding tools or wheels in which the particles are secured in aresinous binder. The finely-divided form of crystalline boron phosphide,because of its abrasive character, is also suitable for use in themanufacture of sand paper and other abrasive products. In this relationthe crystalline material is secured to a backing of paper, cloth, etc.by the use of a suitable glue, cement or resin. Another application forthe crystalline boron phosphide arising from its wear-resistantproperties is as wear plates, for example, in grinders and crushersintended for size reduction of minerals, rocks, etc. and in the grindingof pulp wood in the production of paper.

The chemical inertness and high temperature stability of crystallineboron phosphide makes this a valuable material in the fabrication ofchemical apparatus, such as crucibles and reactors intended particularlyfor use at high temperatures, since this material is resistant againsttemperatures of up to 6,000 F. Agitator arms may also suitably befabricated from this material and burners such as in the manufacture ofacetylene from natural gas, phosphorus pentoxide from phosphorus aretypical examples of elements which may be fabricated from thecrystalline form of boron phosphide. Such burners may also be made inthe form of heat exchangers, since it is well known that burners mustoperate at very high temperatures because of the radiation effects suchas in the combustion of elemental phosphorus to phosphorus pentoxide byair or oxygen.

Another field of application of the crystalline form of boron phosphideis as a nuclear reactor shield in the operation of atomic piles andother reactors in the field of atomic energy.

The crystalline boron phosphide that is made by the process of thepresent invention is particularly advantageous in the manufacture ofsemi-conductors.

It has been found that this crystalline material is characterized by anegative temperature coefficient of resistance which makes the materiala suitable component in the manufacture of various semi-conductorproducts,

particularly for high temperature service. It has been found by opticalmeasurements on cubic crystalline boron phosphide that it has aforbidden energy gap of about 6.8

electron volts. For example, rectifiers, transistors, and otherbarrier-layer devices may be made from the crystalline boron phosphide,together with other crystalline ma terials and with the development ofsuitable controlled impurities if desired, for example, elements ofgroups II or VI of the periodic table.

The semi-conductor properties of the crystalline form of boron phosphideare also shown by the optical characteristics of this cubic crystallinematerial. An unusually eifective field of utilization therefore is anoptical window in test instruments and in guided missiles and spaceships. Such a window may also be fabricated with an external boronphosphide protective layer deposited upon a base of quartz or othertransmissive material. It

has been found that the cubic crystalline form of boron phosphide ischaracterized by an unexpected transmissive power for radiation ofcharacteristic wave lengths (such as from about 1,850 angstroms to about8,000 angstroms). This permits the fabrication of a window which is tobe subjected to high temperature and high pressure conditions, forexample, as an observation port in a furnace or nuclear reactor or as anobservation port for a space ship or missile which is intended toapproach electrical modification of the material which is readilymeasured by conventional electrical procedures. Thus, it is possible toprovide a radiation meter which can withstand extreme conditions ofstrong radiation, for example, in a nuclear reactor, together with hightemperature and pressure without failure of the metal element.

Since the cubic crystalline form of boron phosphide may be fabricated inorder to achieve both dense (e.g. nearly theoretical density) and poroussurfaces, a number of fields of application are based upon thisproperty.

For example, the dense form of crystalline boronphosphide may be used tofabricate a solid nose cone of needle form or blunt form intended for aguided missile. This crystalline boron phosphide is highly resistantagainst heating and thus withstands the attack by erosive gases to whicha missle nose cone is subjected upon re-entry into the atmosphere. Undersuch conditions, speeds of up to 25,000 miles per hour may beencountered, together with surface temperatures in the order of 10,000F.- or above, and it is consequently imperative that a thermally stablematerial be available for this application, even though the extremeconditions may exist for only a short time, such as about 30 seconds.

A number of fabrication methods are available to produce theabove-described manufactured products from crystalline boron phosphide.If it is desired to employ the crystalline material in powder form, oneof the desirable methods is the hot pressing technique in which thepowder is placed in a die of the desired form and subjected to anelevated temperature, for example from 1,000" F. to 6,000 F. for asufficient time to consolidate the crystalline material and effectsuflicient sintering to achieve the desired density. The pressure isgenerally about 500 to 20,000 p.s.i. A flux or bonding agent may also beemployed in this relationship; suitable materials for this purposeinclude one or more of the metals: iron, nickel, cobalt, chromium,niobium, tantalum, titanium, zirconium, tungsten, moylbdenum andhafnium; and the oxides alumina, zirconia, hafnia, silica, beryllia,titania, thoria, as well as combinations of the oxides and combinationswith the said metals. Inorganic compounds having fluxing or bondingproperties, such as the borates -or phosphates, e.g., the alkali boratesand phosphates may of boron phosphide. Another additive which may be employed in the pressing operation is asbestos, since it has been foundthat when the composite article is later subjected to a vacuum heatingor oxidizing condition, such as a combustion gas flame at about 6,000F., the asbestos is burned out or fused, leaving the crystalline boronphos-. phide which is of utility in the use of sweat or transpirationcooling. This method is employed for cooling missile, rocket or spaceship external and internal surfaces which are subjected to hightemperatures. objects having a wall of crystalline boron phosphidepermit the exudation of a liquid, such as water, alcohol or the liquidfuel through the porous wall so that the liquid, upon passing throughthe porous boron phosphide is evaporated to provide an unusuallyefiicient cooling effect.

The above-described porous form of fabricated boron The porous phosphideis also of utility as a filter element, particularly for corrosive uses.Thus, in the fuel system for a rocket or missile, it is necessary to,filter the fuel and/or oxidizing agent in order to avoid clogging theline. This presents a dilficult problem in the case of corrosive agents,such as fuming nitric acid which attacks most metals. However, when aporous boron phosphide filter is inserted in the fuel or oxidant line,this filtering effect is readily accomplished without the dangerofcorrosion or dissolution of the crystalline boron phosphide.

In the hot pressing operation it may also be desirable to control phasechanges of the boron phosphide by the use of specific additives. Forexample, transition temperature changes may be controlled by theaddition of silicon carbide, zinc oxide and other crystalline materialsto aid in the pressing operation.

Cold pressing or indenting of the crystalline boron phosphide is anotherfabrication method which may be employed, particularly with the use of abinder such as sodium silicate for the fabrication of various parts andfittings. The pressure utilized may be up to 200,000 p.s.i. Suitablemetallic additives which may. be employed, to gether with thecrystalline boron phosphide include iron, nickel, cobalt, chromium,niobium, tantalum, titanium, zirconium, tungsten, molybdenum andhafnium, while refractory and insulating oxides, such as alumina,zirconia, hafnia, silica, beryllia, titania, thoria may also be employedsingly or in combination, including combinations with the said metals.The cold pressed material is subsequently treated in various ways, suchas by sintering, reducing or partially oxidizing the fabricated article,in which case the boron phosphide may also undergo a number ofcontrolled modifications. The use of partial oxidation of the coldpressed material also permits the development of porosity, such as bythe employment of additives exemplified by naphthalene and other organiccompounds as well as cork and asbestos, since the heating and oxidationresults in the burning out or transformation of such binder constituentsto a glassy or crystalline matrix, which together with the change in thesaid metals or oxide constituents serves to secure and bond the boronphosphide particles.

If a minor proportion of elemental boron is employed as an additive withthe crystalline boron phosphide in either hot pressing or cold pressing,the fabricated part may be subjected to a phosphorization treatmentinwhich the piece is subjected to the vapor of elemental phosphorus oranother phosphorus compound, for ex ample, phosphine, in order toconsolidate the fabricated part with the transformation of the boronbinding agent to boron phosphide.

If it is desired to make use of the cubic crystalline form of boronphosphide as a hardening element in a metallic base, for example, ironin the production of wear plate, the boron phosphide may be produceddirectly in such a metallic matrix. The base metals which may beemployed in this relationship include the group of aluminum, magnesium,copper, titanium, chromium, manganese, vanadium, zirconium, molybdenum,tantalum, thorium, iron, nickel, lead, tin, antimony, bismuth and Zinc.Articles of this type are useful to withstand wear and abrasion such asin the manufacture of a chute for a sand or other minerals. Another usefor such a reinforced metal is as a battle in a steam turbine. The abovedescribed process for the reaction of a phosphorus source such asferrophosphorus and a boron source such as ferroboron at elevatedtemperature results in the production of the desired cubic crystallineform of boron phosphide which is obtained in dispersed form in the ironmatrix.

Pack diffusion is another method for applying crystalline boronphosphide to desired metallic or ceramic parts. In this method,particles of the crystalline boron phosphide are packed around thedesired metallic or ceramic parts and the entire mixture subjected to ahigh temperature, e.g., about 1500 F. to, 6000 F., for a suitable periodof time to enable diffusion of the boron phosphide to take place intothe desired parts and fittings.

If it is desired to coat or plate the crystalline boron phosphide onvarious sub-strates of metal or refractory parts, particularly whenintricate sections are involved, a flame spraying technique isdesirable. In this method, a high temperature flame such as a reducingflame is provided with finely-divided particles of crystalline boronphosphide so that the impingement of the flame upon the desiredprototype base parts of metal or refractory coats the parts with auniform and dense deposit of the crystalline boron phosphide.

Another method which may be applied is the deposi tion of a coating ofcrystalline boron phosphide by electrophoresis. This method isparticularly suited for pre cision coating of complicated shapes. Metalsand oxides selected from the group consisting of iron, nickel, cobalt,chromium, niobium, tantalum, titanium, zirconium, tungsten, molybdenumand hafnium; and the oxides alumina, zirconia, hafnia, silica, beryllia,titania, thoria, as well as combinations of the oxides and combinationswith the said metals may also be applied in combination with thecrystalline boron phosphide by the electrophoretic method. In thisprocess an aqueous suspension of the crystalline boron phosphide and thedesired metal or oxide is prepared, preferably with particle size rangesof from I to 10 microns. A suspending or dispersing agent such ascarboxymethylcellulose may also be present. The suspension preparationis then deposited upon the prototype of graphite, a metal or a finescreen metal form utilizing a plating voltage of the order of 6 to voltsdirect current. A uniform coating of the boron phosphide, option allywith a metal and oxide therewith of the group set forth above is thusapplied to the base prototype. The coating is subsequently air dried andis then treated by a low temperature hydrogen reduction in the case ofthe metallic oxides. Hydrogen reduction is not necessary with coatingsof the metal powders, and the boron phosphide is unaffected by suchtreatment. The electrophoretic coating is next densified by peening,rolling or by isostatic pressing, the latter method being particularlyconvenient for small items. A final step after densification is asintering of the coating to provide a uniform and strong coating whichis resistant to chemicals and to abrasion.

A mechanical method of deposition which is available for the fabricationof external layers of crystalline boron phosphide is that of slurrydeposition. In this method the finely-divided crystalline boronphosphide is dispersed in a liquid vehicle such as water, optionallywith a dispersing or suspending agent such as carboxymethylcellulose.Additive materials, such as metals, for example, iron, nickel, cobalt,chromium, niobium, tantalum, titanium, zirconium, molybdenum and hafniumand finelydivided refractories, e.g., alumina, zirconia, hafnia, silica,beryllia, titania, and thoria may also be present.

The forms upon which the slurry is to be deposited are made with aporous structure, for example, from metal powders which have beenloosely consolidated to the desired shape or by the use of a fine meshscreen form having the shape of the desired object. Such a porousprototype is suspended in the liquid vehicle which is then subjected tohigh pressures of the order of 10,000 to 50,000 poundsper square inch.Provision is made for the liquid vehicle to be removed from the interiorof the mold or prototype piece which may have an intricate form, or mayconsist of a simple flat plate as may be desired. As a result of theimposition of pressure upon the dispersion of the crystalline boronphosphide, the slurry is uniformly pressedagainst the prototype with theresult that an interlocking crystalline structure is obtained withoutinternal voids or bridges. When the desired thickness of crystallineboron phosphide has thus been formed, the coating may be subjected tofurther mechanical treatment. For example, the coating thus obtained byslurry dispersion may be densified by peening, rolling or isostaticpressing. Finally the deposited coating of crystalline boron phosphide,together with any additives is sintered to consolidate the coating to adense form.

Another method of fabrication which is of utility in forming bodies fromcrystalline boron phosphide is the slip casting technique. In thismethod a slurry is made of the crystalline boron phosphide, togetherwith any desired additive material, such as finely-divided refractories,e.g., alumina, zircom'a, hafnia, silica, beryllia, tatania, and thoria.This mixture is then used in conventional ceramic slip castingtechniques to obtain the desired shapes and fittings in a green formwhich is then fired, packed or sintered to consolidate the crystallineparticles.

The present patent application includes subject matter specificallyclaimed in copending application Serial Number 691,158.

What is claimed is:

1. Process for the production of crystalline boron phosphide whichcomprises contacting a gaseous boron compound with elemental phosphorusand hydrogen at a temperature of at least 1,l F.

2. Process for the preparation of crystalline boron phosphide in a cubiccrystalline form which comprises 10 contacting boron trichloride withelemental phosphorus and hydrogen at a temperature of from 1,100 F. to3,600 F.

3. Process of claim 2 in which the proportions of the reactants are 1mole of boron trichloride with from 1.0 to 1.5 moles of elementalphosphorus and 1.5 to 5 moles of hydrogen.

4. Process for the production of curved shaped objects which comprisescontacting an inert prototype of the desired shape with a gaseousmixture of approximately 1 mole of boron trichloride with 1 mole ofelemental phosphorus and 1.5 moles of hydrogen at a temperature of from1,100 F. to 3,600 F. and coating the said prototype with crystallineboron phosphide resulting from the reaction of the said borontrichloride, phosphorus and hydrogen.

References Cited in the file of this patent UNITED STATES PATENTS2,026,086 Farncomb Dec. 31, 1935 2,467,647 Alexander Apr. 19, 19492,759,861 Collins et a1. Aug. 21, 1956 2,798,989 Welker July 9, 19572,832,672 Fetterley et a1 Apr. 29, 1958 2,886,726 Berger et a1 May 12,1959

1. PROCESS FOR THE PRODUCTION OF CRYSTALLINE BORON PHOSPHIDE WHICHCOMPRISES CONTACTING A GASEOUS BORON COMPOUND WITH ELEMENTAL PHOSPHORUSAND HYDROGEN AT A TEMPERATURE OF A LEAST 1,100*F.